Field of the Invention
The present invention relates to a vibration control apparatus, a lithography apparatus, and an article manufacturing method.
Description of the Related Art
In a lithography apparatus configured to transfer or form ultra-fine patterns, the vibration transmitted from a floor on which the apparatus is mounted to the apparatus may cause degradation of overlay accuracy or resolution (transfer) performance. Hence, in the conventional lithography apparatus, its surface plate, which is the main body thereof, is supported via a vibration control apparatus (vibration isolation apparatus) so as to reduce influence of the floor vibration. The conventional vibration control apparatus includes a gas spring for supporting the surface plate, and further includes a velocity feedback control system that is composed of an acceleration sensor for detecting acceleration of the surface plate and an actuator for applying a force to the surface plate so as to damp the vibration. However, in the case in which the vibration is damped by the feedback control system, the natural frequency of the vibration control apparatus, which depends on the natural frequency of the gas spring, is 3 to 5 Hz at the lowest. Thus, in order to isolate the vibration in a lower frequency, the natural frequency of the vibration control apparatus needs to be reduced.
Japanese Patent Laid-Open No. 2012-97786 discloses a vibration control apparatus including a two-degrees-of-freedom vibration system composed of a reference object and a supporting unit having a mass body for supporting the object. In this vibration control apparatus, a control force is applied to the mass body so that relative displacement between the reference object and the mass body is constant in order to isolate the vibration of the reference object in a lower frequency, and further, a controlled object is positioned at the reference object so as to isolate the vibration of the controlled object in a lower frequency within the servo bandwidth for position control.
Japanese Patent Laid-Open No. 2009-168122 discloses a vibration isolation apparatus including a one-degree-of-freedom vibration system composed of a reference object and a supporting unit for supporting the object. In this vibration isolation apparatus, the reference object is supported by the supporting unit with a low natural frequency so as to isolate the vibration in a lower frequency, and the controlled object is positioned at the reference object so as to isolate the vibration of the object in a lower frequency within the servo bandwidth for position control. In addition, this vibration isolation apparatus further includes a displacement sensor installed therein for detecting displacement of a base structure, and a driving unit located between the controlled object and the base structure applies a control force to the controlled object based on the detected displacement in order to cancel a disturbance force applied to the controlled object due to the displacement of the base structure. Such feedforward control can suppress a disturbance force from being applied to the controlled object due to the displacement of the base structure.
However, in the vibration control apparatus disclosed in Japanese Patent Laid-Open No. 2012-97786, since the controlled object cannot track the reference object in a frequency bandwidth exceeding the servo bandwidth, the vibration isolation performance cannot be improved. In contrast, in the vibration isolation apparatus disclosed in Japanese Patent Laid-Open No. 2009-168122, since a displacement sensor further needs to be installed for detecting displacement of the base structure, the cost will be increased. In addition, when a detecting point of the displacement sensor for measuring a position of the reference object relative to the controlled object for positioning the controlled object at the reference object is different from a detecting point of the displacement sensor for detecting displacement of the base structure, the vibration of the controlled object due to the displacement of the base structure cannot be accurately detected.